Glass sheets making use of a photocatalytic function due to a titanium oxide film are manufactured and are commercially available, as so-called “self-cleaning glass”. As is well known, the photocatalytic function of the titanium oxide film allows organic substances adhered to the surface of a glass sheet to be decomposed and weakens the adherence of the organic substances, thus enabling the organic substances to be washed away by rain water, etc.
Since the refractive index of titanium oxide (which is about 2.5 in the case of anatase type) is higher than the refractive index of glass, the optical reflectance of the glass sheet increases by forming a titanium oxide film on the surface of the glass sheet. Hence, in consideration of an application where such an increase in optical reflectance is particularly undesirable (e.g., use in a solar cell or a greenhouse), a photocatalyst film capable of suppressing the increase in optical reflectance has been proposed. In this photocatalyst film, the refractive index of the film is reduced by adding into the film silicon oxide particles having a lower refractive index than the titanium oxide particles, together with titanium oxide particles.
JP 2006-162711A (Patent Literature 1) discloses, as shown in FIG. 6, a photocatalyst film 11 composed of silicon oxide particles 16 of submicron size to which titanium oxide particles 15 of nano size are adhered (claim 1; and FIG. 1). According to Patent Literature 1, the photocatalyst film 11 is configured to have a reflection suppressing function and an amphipathicity (to achieve a low contact angle with respect to both water and liquid paraffin) together with its original photocatalytic function. The amphipathicity of the photocatalyst film 11 is imparted, as can be explained by the theory of Wenzel, by an increase in surface roughness of the silicon oxide particles 16 caused by the adhesion of titanium oxide particles 15 (paragraphs 0033 to 0036).
For formation of the photocatalyst film disclosed in Patent Literature 1, electrostatic adhesion must be employed. Electrostatic adhesion is a method that requires coating twice for silicon oxide particles and for titanium oxide particles together with electrostatic charging operation for a substrate before film formation. Therefore, this method, when being applied to mass production, is problematic because of its low production efficiency and high production cost.
Electrostatic adhesion uses polycation composed of organic components such as PDDA and PSS for electrifying the substrate, but polycation is removed by subsequent sintering. Therefore, the photocatalyst film disclosed in Patent Literature 1 has a structure basically including no binder, as being different from a photocatalyst film formed by a sol-gel method. Such a photocatalyst film that lacks binders to be interposed between respective particles, or between a substrate and the particles, has insufficient film strength and thus is not of practical use, particularly, when long-term outdoor use is considered.
As described above, the photocatalyst film disclosed in Patent Literature 1 requires a high production cost and is not suitable for long-term use.
JP 2008-264777 A (Patent Literature 2) discloses a photocatalyst film containing silicon oxide particles, titanium oxide particles, and a binder component. This photocatalyst film can be formed by a sol-gel method. The binder component is added to a film forming solution in an amount of less than 10 parts by mass with respect to 100 parts by mass of the total amount of the silicon oxide particles, the titanium oxide particles, and the binder component, with reference to the state of hydrolyzable silicone. However, hydrolyzable silicone remains on the film after undergoing hydrolysis and polycondensation, and thus attention should be paid to the fact that the amount of the binder that forms the film practically should be less than the amount indicated with reference to the hydrolyzable silicone. For example, an indication of 10 parts by mass of hydrolyzable silicone in Table 2 as Example 9 of Patent Literature 2 (as a comparative example) means that the amount of the binder component in the film slightly falls below 10 parts by mass with reference to the state in terms of silicon oxide (SiO2). In Patent Literature 2, the reason why the amount of the binder component is limited is to facilitate introduction of gases to be decomposed into the inside of the photocatalyst film by ensuring more voids between the particles (paragraph 0013). Therefore, Patent Literature 2 asserts that substantially no hydrolyzable silicone is preferably added (paragraph 0026).
In Patent Literature 2, the average particle diameter of titanium oxide particles is adjusted to at least 10 nm but not greater than 100 nm (claim 1), and the average particle diameter of silicon oxide particles (inorganic oxides particles) is adjusted to at least 10 nm but less than 40 nm (claim 4). In this way, the average particle diameter of silicon oxide particles is set such that it should not be equal to or more than 4 times the average particle diameter of titanium oxide particles, in Patent Literature 2. In the section of EXAMPLES in Patent Literature 2, silicon oxide particles having a lower average particle diameter than titanium oxide particles are selected. When silicon oxide particles are smaller than titanium oxide particles, silicon oxide particles can be easily interposed between the substrate and the titanium oxide particles. This is convenient in order to prevent an organic substrate from being eroded by titanium oxide particles, which is required to be considered according to Patent Literature 2 (paragraph 0014).
The photocatalyst film disclosed in Patent Literature 2 also has insufficient film strength because the amount of the binder is small.
JP 2010-134462 A (Patent Literature 3) discloses a photocatalyst film composed of a matrix of titanium dioxide. In the film, silicon oxide particles are embedded in the matrix. This photocatalyst film is formed by a sol-gel method.
The photocatalyst film disclosed in Patent Literature 3 contains silicon oxide particles, but does not contain titanium dioxide particles.
In the section of DETAILED DESCRIPTION OF THE INVENTION in Patent Literature 3, a photocatalyst film produced using silicon oxide particles having average spherical diameters of 10 nm to 15 nm and 18 nm to 30 nm is disclosed (paragraph 0107 and paragraph 0108). Since the silicon oxide particles are excessively small, the photocatalyst film disclosed in Patent Literature 3 has low abrasion resistance and insufficient film strength.
Patent Literature 3 also discloses a photocatalyst film produced using silicon oxide particles in the form of long and thin fibers having an average diameter of 10 nm to 15 nm and a length of 30 nm to 150 nm (paragraph 0111). In the photocatalyst film disclosed in Patent Literature 3, the silicon oxide particles may be in the form of fibers.